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emission of minute particles at a high velocity by a 

 luminous body. When these particles impinge upon 

 the retina of the eye, they produce the sensation of light. 

 According to this view, light ought to travel more quickly 

 in water than in air. Another theory, put forward by 

 Huygens about Newton's time, .and developed later, is 

 that light is due to vibration in an imaginary medium 

 called the ether, believed to prevail throughout all space. 

 Luminous bodies set the ether in vibration, and when 

 these undulations reach the eye they give us the sen- 

 sation of light. On this view, light should travel more 

 slowly in a substance like water than it does in air. 



To test the two theories, therefore, an experiment 

 was required by which the relative velocities of light in 

 air and in water could be determined. By Newton's 

 emission theory the velocity should be greater in water 

 than in air ; while according to Huygens's undulatory 

 theory it should be less. Not until the middle of the 

 nineteenth century was a means found of determining 

 experimentally the velocities of light in water and air 

 in a laboratory. The crucial experiment which would 

 decide which theory was true was performed by a 

 French physicist, Jean Leon Foucault, in 1850 ; and it 

 showed that light, which travels at the rate of about 

 186,000 miles a second in air, travels through water at 

 about three-quarters that velocity. The result of this 

 experiment disposed finally of the emission theory, and 

 re-established Huygens's theory that light is due to 

 very rapid vibrations in a hypothetical ether pervading 

 the universe. 



Fifty years before the crucial determination was made 

 of the velocities of light in water and air, a genius of the 

 first magnitude Dr. Thomas Young had shown that 

 red light is produced by nearly 32,000 ether-waves to 



